This invention relates to the storage of spent nuclear fuel.
Spent nuclear fuel (SNF) emits heat and radiation consisting of high-energy gamma rays or photons, and fast neutrons, which must be contained for many decades in containment apparatus. The containment apparatus usually comprises a canister and an overpack. The overpack can be a transportable overpack or an overpack designed for stationary storage. The canister can be one designed for storage only, or can be designed for transporting, short-term storage, or long term storage, such designs being referred to in the art as multi-purpose canisters (MPC). Canisters loaded with SNF must be transported in a suitable transport overpack and stored in suitable storage overpacks, or a suitable permanent overpack. Canisters are generally designed to hold the SNF in a specific arrangement and to fit within each overpack. Each overpack has a central cavity designed to accommodate a canister, and is designed for shielding the radiation emanating from the canister on a permanent basis. The overpack employs concrete, steel, and other effective materials to shield the radiation emitting from the canister containing the SNF.
The overpack must be ventilated to allow the heat from the SNF within the canister to be removed to the atmosphere. The ventilation system generally comprises air entrance ducts at the bottom of the overpack and hot air exhaust ducts at the top, and a vertical space surrounding the canister within the central cavity of the overpack through which the cooling air flows upward as it is warmed by the canister. The rate of air flow into the lower ducts, up along the space between the overpack and the canister, and out the upper ducts is a function of the flow resistance in the air travel path and the temperature of the canister external surface.
The ventilation ducts must be designed to prevent escape to the surrounding area of significant radiation emitting from the decaying SNF but not to interfere with the flow of air. The state of the duct design art prior to the present invention has been to provide curvilinear or serpentine passageways to insure that radiation does not stream out through the duct openings. However, there are problems with such curvilinear or serpentine design strategies, the first being that they are only partially effective in trapping photons, with typical dose rates on a current design (NAC-MPC) being about 24 milli-rem/hour (mrem/hr). The second problem is that the curvilinear duct designs add to the total resistance to airflow resulting in a corresponding reduced rate of ventilation and corresponding heat removal.
It is an object of the present invention to provide an improved ventilated overpack apparatus and method for storing spent nuclear fuel.
It is a further object of the invention to improve the ventilation of overpacks for SNF canisters while at the same time increasing the rate of trapping photons within the overpack.
These objects, and others which will become apparent from the following disclosure, are achieved by the present invention which comprises in one aspect a ventilated overpack apparatus for containing a spent nuclear fuel canister having ambient air inlet ducts and hot air outlet ducts, and having radiation scattering attenuators in the inlet and outlet ducts. The radiation scattering attenuators are preferably located at the external openings of each of the ventilation ducts.
In another aspect, the invention comprises a method of storing spent nuclear fuel comprising inserting spent nuclear fuel in a canister and inserting the canister in a ventilated overpack having inlet ducts and outlet ducts which have radiation scattering attenuators.
The radiation scattering attenuators are preferably of metal or other material in a sheet or tube form, and can be in an egg crate or cross-hatch configuration, or can be of other configurations such as, for example, several parallel sheets or circles within circles. The radiation scattering attenuators function to reduce the number of photons escaping from the overpack.